Method of producing silicon wafer and silicon wafer

ABSTRACT

The present invention provides a method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 10 to 25 ppma and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment so that a residual interstitial oxygen concentration in the wafer should become 8 ppma or less, and a method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 8 ppma or less and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment to form an oxide precipitate layer in a bulk portion of the wafer, as well as silicon wafers produced by these production methods. Thus, there is provided a DZ-IG silicon wafer in which a DZ layer of high quality is formed, and which can maintain high resistivity even if the wafer is subjected to a heat treatment for device production.

TECHNICAL FIELD

[0001] The present invention relates to a technique by which a DZ-IGsilicon wafer having high resistivity and also having high getteringability can surely be obtained.

BACKGROUND ART

[0002] Silicon wafers of high resistivity produced by the floating zonemethod (FZ method) have conventionally been used for power devices suchas high-voltage power devices and thyristors. However, it is difficultto produce a silicon wafer having a large diameter of 200 mm or more bythe FZ method, and the radial resistivity distribution of usual FZwafers is inferior to that of CZ wafers. Therefore, silicon wafersproduced by the CZ method will be promising in the future, becausewafers of excellent radial resistivity distribution can be produced bythe CZ method, and in addition, wafers of a large size having a diameterof 200 mm or more can sufficiently be produced by the method.

[0003] In recent years, in particular, reduction of parasitic capacityis required in semiconductor devices for mobile communications and thelatest C-MOS devices. For this reason, a silicon wafer of highresistivity and a large diameter comes to be needed. Moreover, theeffect of using a high resistivity substrate on reduction oftransmission loss of signals and parasitic capacity in Schottky barrierdiodes has been reported. Furthermore, although the so-called SOI(Silicon On Insulator) wafer may be used in order to obtain furtherhigher performance of the aforementioned semiconductor devices, it isrequired to use a wafer of high resistivity produced by the CZ method asa base wafer even when semiconductor devices are produced by using theSOI wafer in order to obtain a larger diameter of wafer or solve theproblem of transmission loss of signals or the like.

[0004] However, since the CZ method utilizes a crucible made of quartz,not a small amount of oxygen (interstitial oxygen) is introduced into asilicon crystal. Although each of such oxygen atoms is usuallyelectrically neutral, if they are subjected to a heat treatment at a lowtemperature of around 350 to 500° C., a plurality of them gather torelease electrons and become electrically active oxygen donors.Therefore, if a wafer obtained by the CZ method is subsequentlysubjected to a heat treatment at about 350 to 500° C. in the deviceproduction process and so forth, there arises a problem that resistivityof a high resistivity CZ wafer is reduced due to the formation of theoxygen donors.

[0005] In order to prevent the resistivity reduction due to the aboveoxygen donors and obtain a silicon wafer of high resistivity, methodsfor producing a silicon single crystal having a low interstitial oxygenconcentration from an initial stage of the crystal growth by themagnetic field-applied CZ method (MCZ method) were proposed (refer toJapanese Patent Publication (Kokoku) No. 8-10695 and Japanese PatentLaid-open Publication (Kokai) No. 5-58788). Further, there has also beenproposed a method conversely utilizing the phenomenon of the oxygendonor formation, wherein a P-type silicon wafer of a low impurityconcentration and low oxygen concentration is subjected to a heattreatment at 400 to 500° C. to generate oxygen donors, and P-typeimpurities in the P-type silicon wafer is compensated by these oxygendonors so that the wafer should be converted into N-type to produce anN-type silicon wafer of high resistivity (refer to Japanese PatentPublication No. 8-10695).

[0006] However, a silicon single crystal of a low interstitial oxygenconcentration produced by the MCZ method or the like as mentioned abovesuffers from a drawback that the density of bulk defects generated by aheat treatment in the device production process becomes low, and thussufficient gettering effect will be unlikely to be obtained. In devicesof a high integration degree, it is essential to impart gettering effectby a certain amount of oxygen precipitation.

[0007] Further, the method of obtaining a silicon wafer of N-type bygenerating oxygen donors by a heat treatment and compensating P-typeimpurities in the wafer to convert it into N-type is a complicatedmethod that requires a heat treatment for a long period of time.Moreover, this method cannot provide a P-type silicon wafers. Inaddition, this method also has a drawback that resistivity of wafersobtained by this method may vary depending on a subsequent heattreatment. Furthermore, in this method, in case of high interstitialoxygen concentration, it becomes difficult to control the waferresistivity. Therefore, this method suffers from a drawback that a lowinitial concentration of interstitial oxygen in a silicon wafer must beused, and thus the gettering effect of the wafer becomes low.

[0008] In order to solve these problems, the applicants of the presentapplication proposed, in a previous application (Japanese PatentApplication No. 11-241370, PCT/JP00/01124), a method for producing asilicon wafer, which comprises growing a silicon single crystal ingothaving a resistivity of 100 Ω·cm or more and an initial interstitialoxygen concentration of 10 to 25 ppma (JEIDA: Japan Electronic IndustryDevelopment Association) by the Czochralski method, processing thesilicon single crystal ingot into a wafer, and subjecting the wafer toan oxygen precipitation heat treatment so that a residual interstitialoxygen concentration in the wafer should become 8 ppma or less.According to this method, a CZ wafer of high resistivity of whichresistivity is unlikely to decrease even when the wafer is subjected toa heat treatment for device production. Therefore, if this wafer is usedas, for example, a base wafer of SOI wafer, devices of extremely highperformance for mobile communications can be obtained.

[0009] On the other hand, it is considered that, in order to realize awafer having performance of the same level as the SOI wafer by using abulk wafer, of which production cost is more inexpensive compared withSOI wafer, so to speak “high resistivity DZ-IG wafer” of a structurehaving a DZ layer (Denuded Zone layer) sufficiently made defect free ona surface of such a high resistivity CZ wafer is required. Althoughthere has conventionally been the so-called DZ-IG wafer, which isobtained by subjecting a CZ silicon wafer having usual resistivity to aDZ-IG (Intrinsic Gettering) treatment, there has not been conceived toapply this technique to a high resistivity CZ wafer at all. Therefore,the applicant of the present application also disclosed a method forobtaining a high resistivity DZ-IG wafer by a heat treatment that makesthe aforementioned interstitial oxygen concentration 8 ppma or less inthe previous application (Japanese Patent Application No. 11-241370).

[0010] As the DZ-IG treatment applied to a wafer of usual resistivity, athree-step heat treatment is generally used. Supersaturated oxygens inthe vicinity of a wafer surface are out-diffused by a first step hightemperature heat treatment at 1100° C. or higher, a low temperature heattreatment at around 650° C. is performed as a second step heat treatmentto form oxygen precipitation nuclei, and a moderate temperature heattreatment is performed at about 1000° C. as the third step heattreatment to allow growth of the oxide precipitates. By such athree-step heat treatment, an oxide precipitate region is formed in thewafer, and thus a DZ layer in which oxide precipitates do not exist isformed in the vicinity of surface of the front side or back side.

[0011] Therefore, the applicants of the present application applied thesame heat treatment as the above heat treatment as the heat treatmentfor obtaining an interstitial oxygen concentration of 8 ppma or less. Asa result, it was found that a high resistivity DZ-IG wafer having a highresistivity of 100 Ω·cm or more and having a DZ layer free from crystaldefects near the surface and an oxide precipitate layer in which oxideprecipitates are sufficiently precipitated could be obtained.

[0012] It was considered that such a high resistivity DZ-IG wafer couldsufficiently serve as an alternative of SOI wafers for mobilecommunications. However, subsequent investigations revealed that, ifsuch a DZ-IG wafer was subjected to a heat treatment during the deviceproduction process, the resistivity near the wafer surface was extremelyreduced as the case may be, and thus sufficient high resistivity may notbe obtained.

[0013] Further, it was also found that, although the DZ layer formed bysuch a heat treatment was surely made defect free as for defectsoriginated from oxide precipitates, grown-in defects called COP (CrystalOriginated Particle) were not eliminated and still remained.

[0014] COP is a void of 0.1 μm order size formed by aggregation ofexcessive vacancies during the growth of CZ silicon single crystals, andthe internal surface thereof is covered with a thin oxide film. Further,it is known that, if a device is formed on a portion where such grown-indefects exist, device characteristics such as oxide dielectric breakdownvoltage are degraded.

DISCLOSURE OF THE INVENTION

[0015] The present invention was accomplished in order to solve theseproblems, and its object is to provide a method for producing a siliconwafer in which a DZ layer of high quality made defect free not only foroxide precipitates but also for COPs is formed in the vicinity of thewafer surface, oxide precipitates are formed in the bulk portion at asufficient density and thereby high gettering ability can be obtained,and which can maintain high resistivity even after the wafer issubjected to a heat treatment for device production, and thereby providea high resistivity DZ-IG wafer of high quality at a thus-far unknownlevel, which can serve as an alternative of SOI wafer for mobilecommunications.

[0016] In order to achieve the aforementioned object, the presentinvention provides a method for producing a silicon wafer, whichcomprises growing a silicon single crystal ingot having a resistivity of100 Ω·cm or more and an initial interstitial oxygen concentration of 10to 25 ppma and doped with nitrogen by the Czochralski method, processingthe silicon single crystal ingot into a wafer, and subjecting the waferto a heat treatment so that a residual interstitial oxygen concentrationin the wafer should become 8 ppma or less.

[0017] If nitrogen is doped in a silicon single crystal as describedabove, sizes of grown-in defects (COPs) become small, and it becomeseasy to eliminate them by a heat treatment. In addition, formation andgrowth of oxygen precipitation nuclei can be attained to a certaindegree during the crystal growth. Thus, it becomes possible to form a DZlayer of high quality by a heat treatment at a temperature lower thanthat of the conventional DZ-IG treatment by a three-step heat treatment(formation of DZ layer (high temperature)+formation of precipitationnuclei (low temperature)+growth of precipitates (moderate temperature)),and it also becomes possible to grow oxide precipitates of a sufficientdensity in the bulk portion by a heat treatment for a short period oftime. Therefore, the transition region between the DZ layer and theoxide precipitate region can be made to have a narrow and sharp profile,and the amount of interstitial oxygen in the whole transition region canbe made small. Thus, the influence of oxygen acting as donor can bereduced. Furthermore, it also becomes possible to enlarge the acceptablerange of the initial interstitial oxygen concentration that can providean interstitial oxygen concentration of 8 ppma (JEIDA: Japan ElectronicIndustry Development Association Standard) or less for a specific heattreatment.

[0018] The present invention also provides a method for producing asilicon wafer, which comprises growing a silicon single crystal ingothaving a resistivity of 100 Ω·cm or more and an initial interstitialoxygen concentration of 8 ppma or less and doped with nitrogen by theCzochralski method, processing the silicon single crystal ingot into awafer, and subjecting the wafer to a heat treatment to form an oxideprecipitate layer in a bulk portion of the wafer.

[0019] If nitrogen is doped in a silicon single crystal as describedabove, the oxygen precipitation is more promoted when the wafer issubjected to an oxygen precipitation heat treatment compared with awafer not doped with nitrogen, even if the wafer is a wafer of lowoxygen content having an initial interstitial oxygen concentration of 8ppma or less, and thus it becomes possible to form oxide precipitates ata sufficient density. Moreover, since the wafer contains oxygen at a lowconcentration, the oxide films on the internal surfaces of COPs formedduring the crystal growth become thin, and it becomes easy to eliminateCOPs by a heat treatment. For these reasons, it becomes possible to forma DZ layer of high quality and oxide precipitates at a sufficientdensity in the bulk portion by a heat treatment at a relatively lowertemperature for a shorter period of time compared with those used in theconventional techniques. Furthermore, since the interstitial oxygenconcentration is originally 8 ppma or less, substantially no fluctuationof resistivity due to oxygen acting as donor is caused in the deviceproduction process. Moreover, the method also has an advantage that theproblem of low resistance of the wafer to slip dislocations generated bya heat treatment due to the low oxygen concentration in a wafer notdoped with nitrogen can also be covered by the use of the milder heattreatment conditions (low temperature and short time).

[0020] In the aforementioned methods, nitrogen is preferably doped at aconcentration of 1×10¹² to 5×10¹⁵ number/cm³.

[0021] This is because, if the nitrogen concentration is less than1×10¹² number/cm³, the effect is not so remarkable compared with a casenot using the nitrogen doping, and if it exceeds 5×10¹⁵ number/cm³, thesingle crystallization during the pulling of the crystal may beinhibited, or it may make continuous operation unstable. Morepreferably, the nitrogen concentration should be less than 1×10¹⁴number/cm³. This is because, if the nitrogen concentration is 1×10¹⁴number/cm³ or more, the amount of oxygen-nitrogen donors formed by aheat treatment at around 600° C. increases, and they may reduce theresistivity. That is, it is known that about 10% of the doped nitrogencontributes to the formation of oxygen-nitrogen donors, and a dopingamount of 1×10¹⁴ number/cm³ may form 1×10¹³ number/cm³ ofoxygen-nitrogen donors. If all of these donors are activated, there iscaused fluctuation of the resistivity in the order of several hundredsΩ·cm. However, it can be conversely said that, if the amount of thegenerated donors is the above level or less, they show substantially noinfluence.

[0022] Further, in the aforementioned method, the heat treatment ispreferably performed at a temperature of 1000 to 1200° C. for 1 to 20hours in hydrogen gas, argon gas or a mixed gas atmosphere of hydrogengas and argon gas.

[0023] If the heat treatment is performed in hydrogen gas, argon gas ora mixed gas atmosphere thereof as described above, grown-in defects atthe wafer surface and in the vicinity of the wafer surface can beeffectively eliminated, and the oxide precipitates in the bulk portioncan be grown at the same time. In this case, if the heat treatmenttemperature is lower than 1000° C., a heat treatment for a long periodof time exceeding 20 hours is required in order to sufficientlyeliminate the grown-in defects. Further, although the grown-in defectscan be sufficiently eliminated by a heat treatment for about 1 hour at aheat treatment temperature of 1200° C., if the temperature exceeds 1200°C., the oxygen precipitation nuclei formed by the effect of the nitrogendoping during the crystal growth become likely to melt again, and thusit becomes difficult to obtain a sufficient oxide precipitate densityafter the heat treatment. Therefore, the heat treatment temperature ispreferably 1000 to 1200° C.

[0024] A silicon wafer produced by the production method of the presentinvention described above has a high resistivity of 100 Ω·cm or more anda low interstitial oxygen concentration of 8 ppma or less. Therefore, itcan be a high resistivity DZ-IG wafer of high quality in whichresistivity is not reduced by oxygen acting as donor during the deviceproduction process, and which contains substantially no grown-in defectin the DZ layer near the wafer surface.

[0025] As explained above, according to the present invention, there canbe obtained a CZ silicon wafer in which fluctuation of resistivity dueto interstitial oxygen acting as donor is suppressed even after thewafer is subjected to a heat treatment for device production. Thiseffect is extremely effective for a high resistivity CZ wafer having aresistivity of 100 Ω·cm or more, and it enables use of the wafer as analternative of SOI wafer for mobile communications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a graph representing experimental results showing therelationship between the initial interstitial oxygen concentration andthe residual interstitial oxygen concentration of a CZ wafer subjectedto a usual oxygen precipitation heat treatment.

[0027]FIG. 2 is a graph representing experimental results showing therelationship between the initial interstitial oxygen concentration andthe density of precipitates of a CZ wafer subjected to a usual oxygenprecipitation heat treatment.

[0028]FIG. 3 is a graph showing the relationship between the depth fromsurface and the resistivities before and after heat treatment in aconventional silicon wafer.

[0029]FIG. 4 is a graph showing the relationship between the depth fromsurface and the absolute value of oxygen concentration in a conventionalsilicon wafer.

[0030]FIG. 5 is a schematic view showing precipitate distribution alongthe depth direction in a conventional silicon wafer.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Hereafter, the present invention will be explained in detail.

[0032] As described above, in order to obtain a “high resistivity DZ-IGwafer” that can realize, as a bulk wafer, performance at a levelequivalent to that of an SOI wafer for mobile communications utilizing ahigh resistivity wafer as the base wafer, the inventors of the presentinvention applied a three-step heat treatment usually performed as aheat treatment for obtaining an interstitial oxygen concentration of 8ppma or less to a CZ silicon wafer of high resistivity as a trial. Theheat treatment of the third step for growing oxide precipitates wasperformed by divided two stages at temperatures of 800° C. and 1000° C.

[0033] As a result, in the wafer immediately after the three-step heattreatment, a DZ layer was formed in the vicinity of the wafer surfacewhile the high resistivity was maintained, and an IG layer (oxideprecipitate region) was formed in the bulk portion. Thus, a desired highresistivity DZ-IG wafer was obtained (FIG. 3(a)). However, when a heattreatment simulating a device production heat treatment was applied tothis wafer, it was found that the resistivity might extremely decreasedin the vicinity of the wafer surface as the case may be (FIG. 3(b)).

[0034] It was expected that the resistivity decrease was caused becauseinterstitial oxygen existing in the wafer became donor. Therefore, theinventors of the present invention measured and examined distribution ofabsolute value of oxygen concentration along the depth direction in awafer after the three-step heat treatment, in which the resistivitydecreased, by using a secondary ion mass spectroscopy (SIMS) apparatus(FIG. 4). Further, the wafer was subjected to angle polishing andpreferential etching, and then distribution of oxide precipitates (etchpits) along the depth direction was observed. The results areschematically shown in FIG. 5.

[0035] From the results shown in FIGS. 4 and 5, it can be seen that aregion of about 20 μm from the surface is the DZ layer, a deeper regionof a depth of about 30 μm or more from the surface is the oxideprecipitate layer, and a region between them of a depth of about 20 to30 μm from the surface is the transition region (a region that does notfully become a DZ layer, in which a few oxide precipitates exist) in thewafer referred to in FIG. 4. A region around the transition regioncorresponded to a region in which resistivity was extremely decreasedafter the heat treatment simulating a device production heat treatment.When the interstitial oxygen concentration in this region was measuredlater by infrared absorption spectroscopy, it was found to be a portionwhere the interstitial oxygen concentration exceeded 8 ppma (4×10¹⁷atoms/cm³).

[0036] Furthermore, when COPs in a region of a depth of severalmicrometers from the surface were measured by using a particle counterbefore and after the three-step heat treatment, there is almost nochange was observed, and thus it was confirmed that COPs remained in thewafer surface portion.

[0037] That is, it is considered that, even if the interstitial oxygenin the DZ layer near the surface is out-diffused, and the interstitialoxygen concentration in the bulk portion becomes sufficiently low due toprecipitation of oxygen as oxide precipitates, the interstitial oxygenconcentration is still high in the transition region between them evenafter the three-step heat treatment, and therefore the resistivity isdecreased by oxygen becoming donor.

[0038] Therefore, it was considered that, for the purpose of surelyobtaining a high resistivity DZ-IG wafer, the interstitial oxygenconcentration of not only the DZ layer and the oxide precipitate layerbut also the transition region between the both should be made to be 8ppma or less, or if the aforementioned transition region could be madeto have a width as narrow as possible and a profile as sharp aspossible, the amount of interstitial oxygen as the whole transitionregion would become small, and thus the influence of oxygen that becamedonor could also be made small. In addition, it was also considered thatit was necessary to decrease COPs in the DZ layer.

[0039] Therefore, the inventors of the present invention investigatedproduction conditions for a silicon wafer that satisfy theserequirements. As a result, they conceived doping of a silicon singlecrystal with nitrogen during the growth of the crystal by the CZ method.That is, it has been pointed out that, if nitrogen is doped in a siliconsingle crystal, the aggregation of oxygen atoms in the silicon ispromoted, and thus the oxide precipitate density increases (T. Abe andH. Takeno, Mat. Res. Soc. Symp. Proc. Vol. 262, 3, 1992). It is thoughtthat this effect is obtained because the aggregation process of oxygenatoms is shifted from that consisting of homogenous nucleus formation tothat consisting of heterogenous nucleus formation utilizing nitrogenimpurities as nuclei. Furthermore, it is also known that, if nitrogen isdoped in a single crystal, sizes of crystal defects such as COPs becomesmall.

[0040] Therefore, if a silicon single crystal is doped with nitrogenduring the growth thereof, formation and growth of oxygen precipitationnuclei can be attained to a certain extent during the crystal growth,thus it is expected that the precipitation of interstitial oxygen ispromoted also in the transition region between the DZ layer and theoxide precipitate layer to sufficiently reduce the interstitial oxygenconcentration, and it is considered that the width of the transitionregion can be narrowed so that the region should have a sharp profile.Further, it is considered that, since sizes of grown-in defects such asCOPs are made small by the nitrogen doping, it becomes easier toeliminate them by a subsequent heat treatment. Therefore, the inventorsof the present invention performed the following experiments concerningnitrogen-doped CZ wafers.

EXPERIMENTAL EXAMPLE 1

[0041]FIG. 1 shows experimental results representing relationshipbetween the initial interstitial oxygen concentration and the residualinterstitial oxygen concentration in various CZ wafers having an initialinterstitial oxygen concentration of 8 to 21 ppma after they weresubjected to usual heat treatments for oxygen precipitation, i.e., aheat treatment at 780° C. for 3 hours and a heat treatment at 1000° C.for 16 hours under a nitrogen atmosphere (containing 3% of oxygen). Twokinds of the nitrogen concentrations, i.e., 1×10¹³ to 9×10¹³ number/cm³and no nitrogen doping, were used.

[0042] From the results shown in FIG. 1, it can be seen that if nitrogenis not doped, the residual interstitial oxygen concentration cannot bemade 8 ppma or less after the aforementioned heat treatments unless theinitial interstitial oxygen concentration is 19 ppma or more, whereas ifnitrogen is doped, the range of acceptable initial interstitial oxygenconcentration is enlarged to the range of 15 ppma or more. Further, itwas confirmed that, by using a longer heat treatment time, the range ofacceptable initial interstitial oxygen concentration could be enlargedto the range of 10 ppma or more.

EXPERIMENTAL EXAMPLE 2

[0043]FIG. 2 shows experimental results representing relationshipbetween the initial interstitial oxygen concentration and the density ofprecipitates in various CZ wafers having an initial interstitial oxygenconcentration of 4 to 19 ppma after they are subjected to usual heattreatments for oxygen precipitation, i.e., a heat treatment at 780° C.for 3 hours and a heat treatment at 1000° C. for 16 hours under anitrogen atmosphere (containing 3% of oxygen), angle polishing andpreferential etching, in which the measured density of precipitates inthe oxygen precipitation region was converted into volume density. Twokinds of the nitrogen concentrations, i.e., 1×10¹³ to 9×10¹³ number/cm³and no nitrogen doping, were used.

[0044] From the results shown in FIG. 2, it can be seen that the oxideprecipitate density is markedly increased by the nitrogen doping, evenif the initial interstitial oxygen concentration is the same. It can beseen that, in particular, even with a low initial interstitial oxygenconcentration of 8 ppma or less, a precipitate density of 1×10⁸number/cm³ can be obtained by the nitrogen doping.

[0045] Based on the results shown in FIG. 1 mentioned above, it isconsidered that it is expected that oxide precipitates are notsubstantially formed in a non-nitrogen-doped wafer of a low oxygenconcentration having an initial interstitial oxygen concentration of 8ppma or less, but when nitrogen was doped, oxygen precipitation nuclei(oxide precipitates) stable at a high temperature had been alreadyformed in the as-grown state, and therefore oxide precipitates havingsizes detectable as oxide precipitates were obtained even by anextremely small degree of nucleus growth in such an extent that thegrowth could not be detected as an amount of precipitated oxygen afterthe subsequent heat treatment.

[0046] From these results, it is expected that, as for wafers obtainedfrom a single crystal having an initial interstitial oxygenconcentration of about 10 to 25 ppma, if the crystal is grown withnitrogen doping, the oxygen precipitation is promoted during thesubsequent heat treatment for oxygen precipitation, thus the residualinterstitial oxygen concentration can be reduced also in the transitionregion, and a sufficiently narrow and sharp profile of the transitionregion can be obtained. Further, it can be seen that, as also for wafersobtained from a single crystal having an initial interstitial oxygenconcentration of 8 ppma or less, if nitrogen is doped, sufficient oxygenprecipitation can be obtained by the subsequent heat treatment foroxygen, and sufficient gettering effect can be secured.

[0047] Further, although the quality of the DZ layer, especially thepresence or absence of COP, was not evaluated in the experimentsdescribed above, it is known that sizes of COPs are made smaller bynitrogen doping, and thus it becomes easier to eliminate them by a heattreatment. Therefore, the inventors of the present invention consideredthat, if elimination of COPs and formation of oxide precipitates wereperformed simultaneously under conditions under which COPs were easilyeliminated, and oxide precipitates are sufficiently formed, i.e., byusing a temperature not so high as the DZ formation heat treatment ofthe conventional three-step heat treatment (high temperature heattreatment) instead of the heat treatments used in the aforementionedexperiments, the transition region could be made to have a narrow widthand a sharp profile, as a result, high resistivity could be maintained,and the desired CZ silicon wafer could eventually be obtained, and theyaccomplished the present invention.

[0048] The present inventions will be further explained hereafter.However, the present invention is not limited by these explanations.

[0049] First, a silicon single crystal ingot is pulled by the known CZmethod or the known MCZ method where a single crystal is pulled while amagnetic field is applied to a melt in the CZ method to controlconvection of the silicon melt, so that the silicon single crystal ingotshould have a desired high resistivity of 100 Ω·cm or more and aninitial interstitial oxygen concentration of 10 to 25 ppma. Thesepulling methods are methods comprising bringing a seed crystal intocontact with a melt of polycrystalline silicon raw material contained ina quartz crucible and slowly pulling the seed crystal with rotation toallow growth of a single crystal ingot of a desired diameter. A desiredinitial interstitial oxygen concentration can be obtained by usingconventional techniques. For example, a crystal having a desired oxygenconcentration can be obtained by suitably adjusting parameters such asrotational speed of the crucible, flow rate of introduced gas,atmospheric pressure, temperature distribution and convection of siliconmelt and strength of the magnetic field to be applied.

[0050] In order to pull a silicon single crystal having an initialinterstitial oxygen concentration of 8 ppma or less (also referred to as“low oxygen concentration” hereafter), the parameters to be controlledduring the crystal growth are similar to those for the case where theinterstitial oxygen concentration is 10 to 25 ppma (also referred to as“high oxygen concentration” hereafter) is pulled as mentioned above.However, in order to stably pull such a crystal of low oxygenconcentration, the MCZ method is usually used.

[0051] The simultaneous doping with nitrogen as well as oxygen can beeasily performed by preliminarily adding nitride such as wafers havingnitride films into the raw material polycrystal contained in a quartzcrucible. The concentration of nitrogen to be doped in a pulled crystalcan be calculated from the amount of nitride introduced into the rawmaterial polycrystal or crucible, the segregation coefficient ofnitrogen and so forth.

[0052] The CZ silicon single crystal ingot obtained as described aboveis sliced by using a cutting machine such as a wire saw or innerdiameter slicer, and subjected to steps of chamfering, lapping, etching,polishing and so forth to be processed into CZ silicon single crystalwafers according to conventional techniques. Of course, these steps aremere examples, and there may be used various other steps such ascleaning step and heat treatment step. Further, the steps are used withsuitable modification including the alteration of the order of steps,omission of some steps and so forth according to the purpose.

[0053] Then, to a wafer of high oxygen concentration, a heat treatmentthat provides a residual interstitial oxygen concentration of 8 ppma orless is applied. The heat treatment that provides a residualinterstitial oxygen concentration of 8 ppma or less used in this casecannot be necessarily specified, because the residual interstitialoxygen concentration varies depending on the initial interstitial oxygenconcentration and thermal history during crystal growth of a wafer to besubjected to a heat treatment. However, it can be determined byexperiments according to the initial interstitial oxygen concentration,thermal history and so forth.

[0054] In addition, in the present invention, not only the formation ofoxide precipitates by a heat treatment, but also elimination of COPs bya heat treatment must be considered. The heat treatment for eliminatingCOPs is preferably a heat treatment at a high temperature in hydrogengas, argon gas or a mixed gas thereof. However, at an unduly hightemperature, there arise problems that oxide precipitates becomeunlikely to be formed, and the width of the transition region isbroadened. Therefore, the heat treatment temperature is preferably 1000to 1200° C. Even with such a relatively low temperature, COPs can besufficiently eliminated, because the sizes of COPs are made small by theeffect of the nitrogen doping, and interstitial oxygen can also bereduced by out-diffusion thereof. Further, if hydrogen gas, argon gas ora mixed gas thereof is used, the out-diffusion profile of oxygenabruptly changes at the wafer surface, and thus a transition regionhaving a sharper profile can be formed.

[0055] The aforementioned heat treatment can also be dividedlyperformed, for example, with two stages at 1200° C. and 1000° C., tosufficiently eliminate COPs at a high temperature of the first stage,and then sufficiently grow oxide precipitates at a low temperature ofthe second stage.

[0056] On the other hand, as for a wafer of low oxygen concentration,change of resistivity depending on the width of the transition regionneed not be considered, COPs can be easily eliminated by a heattreatment at a temperature as high as possible within a range thatprovides sufficient formation of oxide precipitates. However, in orderto obtain sufficient oxygen precipitates, the temperature is preferably1000 to 1200° C.

[0057] These heat treatments can be performed by using a usual verticaltype furnace or a usual horizontal type furnace (diffusion furnace),which enables a simultaneous heat treatment of many wafers.

[0058] The present invention will be specifically explained hereafterwith reference to the following examples of the present invention andcomparative examples. However, the present invention is not limited bythese.

EXAMPLE 1

[0059] Silicon wafers having nitride films were introduced into rawmaterial polycrystal, and a nitrogen-doped silicon single crystal waspulled by the CZ method (without applying magnetic field). The siliconsingle crystal was processed by a usual method to produce a CZ siliconwafer having a diameter of 200 mm, an initial interstitial concentrationof 18 ppma (JEIDA), a nitrogen concentration of 8×10¹³ number/cm³(calculated value) and a resistivity of 2500 Ω·cm.

[0060] This wafer was subjected to a heat treatment at 1100° C. for 2hours and a heat treatment at 1000° C. for 16 hours under a 100% argonatmosphere and further subjected to heat treatments simulating a deviceproduction process (heat treatments at 1200° C. for 1 hour and at 450°C. for 5 hours) under a nitrogen atmosphere (mixed with 3% of oxygen) ina vertical type heat treatment furnace. For the silicon wafer obtainedas described above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

[0061] Thereafter, the wafer after the heat treatments was subjected toangle lapping, and then resistivity was measured by the spreadingresistance measurement method for a portion from the surface to a depthof 100 μm. As a result, it was confirmed that every region in the waferhad a resistivity of 2000 Ω·cm or more. Subsequently, the surfaceundergone angle polishing was subjected to preferential etching andobserved by using a optical microscope. As a result, the transitionregion had a narrow width of about 5 μm, i.e., a sharp profile, and thedefect density in the oxygen precipitation region was a sufficientvalue, i.e., 5 to 8×10⁹ number/cm³. That is, it can be seen that thesilicon wafer of Example 1 was a DZ-IG wafer scarcely influenced by theresistivity reduction due to the oxygen donor and having sufficientgettering ability.

[0062] Further, the surface of the wafer after the heat treatments waspolished by about 3 μm, and density of COPs existing on the polishedsurface and having a size of 0.12 μm or more as the diameter weremeasured by using a particle counter (SP1, produced by KLA TencorCorporation). The measured COP density was an extremely low density,i.e., 0.06 number/cm².

COMPARATIVE EXAMPLE 1

[0063] A silicon single crystal was pulled with the same conditions asthose of Example 1 except that nitrogen was not doped, and processed bya usual method to produce a CZ silicon wafer having a diameter of 200mm, an initial interstitial concentration of 18 ppma (JEIDA) and aresistivity of 2500 Ω·cm.

[0064] This wafer was subjected to a three-step heat treatment (DZ-IGtreatment) comprising a heat treatment at 1150° C. for 4 hours, a heattreatment at 650° C. for 6 hours and a heat treatment at 1000° C. for 16hours under a nitrogen atmosphere (mixed with 3% of oxygen) and furthersubjected to heat treatments simulating a device production process(heat treatments at 1200° C. for 1 hour and at 450° C. for 5 hours) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

[0065] Thereafter, the wafer after the heat treatments was subjected toangle lapping, and then resistivity was measured by the spreadingresistance measurement method for a portion from the surface to a depthof 100 μm. As a result, it was confirmed that the resistivity wasdecreased to ten and several Ω·cm in a region of a depth of 20 to 40 μmfrom the surface. Subsequently, the surface undergone angle polishingwas subjected to preferential etching and observed by using a opicalmicroscope. As a result, the region in which resistivity was decreasedsubstantially corresponded to the transition region. The interstitialoxygen concentration was confirmed again by infrared absorptionspectroscopy only for the transition region, it was found that theregion is a portion where the interstitial oxygen concentration exceeded8 ppma (4×10¹⁷ atom/cm³). Based on this, it is considered that theamount of interstitial oxygen becoming donor was large in that portion,thus the conductivity type was reversed from p-type into n-type, andtherefore the resistivity was further decreased. The measurement of theinterstitial oxygen concentration in the transition region by infraredabsorption spectroscopy can be performed by, for example, a measurementmethod of using a bonded wafer obtained by eliminating the DZ layer bypolishing, bonding the obtained surface to an FZ wafer and eliminatingthe oxide precipitate layer.

[0066] Further, the surface of the wafer after the heat treatments waspolished by about 3 μm, and density of COPs existing on the polishedsurface and having a size of 0.12 μm or more as the diameter weremeasured by using a particle counter (SP1, produced by KLA TencorCorporation). The measured COP density was 4.3 number/cm², which wassubstantially the same density as the density before the heattreatments.

EXAMPLE 2

[0067] Silicon wafers having nitride films were introduced raw materialpolycrystal, and a nitrogen-doped silicon single crystal was pulled bythe MCZ method. The silicon single crystal was processed by a usualmethod to produce a CZ silicon wafer having a diameter of 200 mm, aninitial interstitial concentration of 6 ppma (JEIDA), a nitrogenconcentration of 9×10¹³ number/cm³ (calculated value) and a resistivityof 1500 Ω·cm.

[0068] This wafer was subjected to a heat treatment at 800° C. for 4hours under a nitrogen atmosphere and a heat treatment at 1100° C. for16 hours under an argon atmosphere (mixed with 3% of hydrogen) andfurther subjected to heat treatments simulating a device productionprocess (heat treatments at 1200° C. for 1 hour and at 450° C. for 5hours) in a vertical type heat treatment furnace. For the silicon waferobtained as described above, the residual interstitial oxygenconcentration was measured by infrared absorption spectroscopy for thewhole wafer, and it was confirmed that the residual interstitial oxygenconcentration in the wafer was 8 ppma or less.

[0069] Thereafter, the wafer after the heat treatments was subjected toangle lapping, and resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that every region in the wafer had aresistivity of 1000 Ω·cm or more. Subsequently, the surface undergoneangle polishing was subjected to preferential etching and observed byusing a optical microscope. As a result, the defect density in the oxideprecipitate region was a sufficient value, i.e., 1 to 5×10⁸ number/cm³.That is, it can be seen that the silicon wafer of Example 2 was also aDZ-IG wafer scarcely influenced by the resistivity reduction due to theoxygen donor and having sufficient gettering ability, even though thesilicon single crystal was grown with a low oxygen concentration.

[0070] Further, the surface of the wafer after the heat treatments waspolished by about 3 μm, and density of COPs existing on the polishedsurface and having a size of 0.12 μm or more as the diameter weremeasured by using a particle counter (SP1, produced by KLA TencorCorporation). The measured COP density was extremely low density, i.e.,0.1 number/cm².

COMPARATIVE EXAMPLE 2

[0071] A silicon single crystal was pulled with the same conditions asthose of Example 2 except that nitrogen was not doped, and processed bya usual method to produce a CZ silicon wafer having a diameter of 200mm, an initial interstitial concentration of 6 ppma (JEIDA) and aresistivity of 1500 Ω·cm.

[0072] This wafer was subjected to a three-step heat treatment (DZ-IGtreatment) comprising a heat treatment at 1150° C. for 4 hours, a heattreatment at 650° C. for 6 hours and a heat treatment at 1000° C. for 16hours under a nitrogen atmosphere (mixed with 3% of oxygen) and furthersubjected to heat treatments simulating a device production process(heat treatments at 1200° C. for 1 hour and at 450° C. for 5 hours) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

[0073] Thereafter, the wafer after the heat treatments was subjected toangle lapping, and resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that every region in the wafer had aresistivity of 1000 Ω·cm or more. However, when the surface undergoneangle polishing was subjected to preferential etching and observed byusing a optical microscope, the defect density in the oxide precipitateregion was found to be extremely low, i.e., 1 to 5×10⁶ number/cm³. Thatis, it can be seen that, although the silicon wafer of ComparativeExample 2 was a wafer not substantially influenced by the resistivityreduction due to the oxygen donor, it was a wafer that did not havesufficient gettering ability, since the silicon single crystal was grownwith a low oxygen concentration, and thus oxygen precipitation wasunlikely to occur in the bulk portion.

[0074] Further, the surface of the wafer after the heat treatments waspolished by about 3 μm, and density of COPs existing on the polishedsurface and having a size of 0.12 μm or more as the diameter weremeasured by using a particle counter (SP1, produced by KLA TencorCorporation). The measured COP density was 4.0 number/cm², which wassubstantially the same density as the density before the heattreatments.

[0075] The present invention is not limited to the embodiments describedabove. The above-described embodiments are mere examples, and thosehaving the substantially same configuration as that described in theappended claims and providing the similar functions and advantages areincluded in the scope of the present invention.

1. A method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 10 to 25 ppma and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment so that a residual interstitial oxygen concentration in the wafer should become 8 ppma or less.
 2. A method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 8 ppma or less and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment to form an oxide precipitate layer in a bulk portion of the wafer.
 3. The method according to claim 1 or 2, wherein nitrogen is doped at a concentration of 1×10¹² to 5×10¹⁵ number/cm³.
 4. The method according to any one of claims 1 to 3, wherein the heat treatment is performed at a temperature of 1000 to 1200° C. for 1 to 20 hours in hydrogen gas, argon gas or a mixed gas atmosphere of hydrogen gas and argon gas.
 5. A silicon wafer produced by the production method according to any one of claims 1 to
 4. 